[turbofan] Add dedicated test for check constant folding.

This adds a dedicated test to make sure we don't try constant folding on
checks (in this case CheckTaggedPointer), which would generate invalid
code as we removing checks that guard the constant without knowing
whether it's safe to do so.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2087153002
Cr-Commit-Position: refs/heads/master@{#37168}

src/compiler/js-typed-lowering.cc

@@ -1853,6 +1853,10 @@ Reduction JSTypedLowering::Reduce(Node* node) {
   // result value and can simply replace the node if it's eliminable.
   if (!NodeProperties::IsConstant(node) && NodeProperties::IsTyped(node) &&
       node->op()->HasProperty(Operator::kEliminatable)) {
+    // We can only constant-fold nodes here, that are known to not cause any
+    // side-effect, may it be a JavaScript observable side-effect or a possible
+    // eager deoptimization exit (i.e. {node} has an operator that doesn't have
+    // the Operator::kNoDeopt property).
     Type* upper = NodeProperties::GetType(node);
     if (upper->IsInhabited()) {
       if (upper->IsConstant()) {

test/mjsunit/compiler/dont-constant-fold-deopting-checks.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --allow-natives-syntax
+
+function bar(a) { a[0](true); }
+function foo(a) { return bar(1); }
+%OptimizeFunctionOnNextCall(foo);
+assertThrows(function() {bar([foo])}, TypeError);